1. Field of the Invention
The invention relates to audio filter circuits and comprises a stable, low noise audio filter circuit that is switchable to provide either single-ended or double-ended output.
2. Background Information
Audio filter circuits are frequently interposed between an audio source and audio output devices such as speakers, tape recorders, and the like. Such circuits provide certain desirable functionality and characteristics for the audio signal passing through them, such as gain control, noise reduction, and driving impedance matching, among other capabilities.
Digital to analog (DAC) converters are frequently used in sound generating, recording and processing systems to convert signals that are generated or processed in the digital domain into signals in the analog domain that are to be recorded on analog tape recorders or simply played through loudspeakers. The output of such converters frequently must be carefully filtered in order to remove undesirable high frequency components, both image and noise, without degrading the signal content. For example, some DAC""s use a high-frequency, switched-capacitor, low pass filter to reduce high frequency noise at the output. Such a filter is economical, but nonetheless leaves a noise residue that can degrade the quality of a sound signal that subsequently is recorded or that is played through a loudspeaker.
Audio filter circuits may provide either single-ended or double-ended output, depending on the application. In circuits which provide both, it is difficult to provide flexible switching over a range of circuit gains while maintaining essentially the same noise and gain characteristics in the output circuit. Since noise from the DAC differential (double-ended, balanced) outputs is partially correlated, it can be reduced by subtraction using a differential amplifier.
Accordingly, it is an object of the invention to provide an improved audio filter circuit.
Further, it is an object of the invention to provide an improved audio filter circuit having a high signal to noise ratio.
Another object of the invention is to provide an improved audio filter circuit which enables either single-ended or double-ended output.
Still another object of the invention is to provide an improved audio filter circuit which enables either single-ended or double-ended output and which maintains essentially the same noise and gain characteristics independent of the output mode.
Yet another object of the invention is to provide an improved dual mode audio filter circuit having selectable gain and providing essentially the same noise and gain characteristics independent of the output mode.
In accordance with the present invention, an audio filter circuit is formed from a pair of cross-coupled, symmetric filters, each filter having a first stage comprising a single-pole, inverting low pass filter followed by a second stage comprising a two-pole, non-inverting Sallen-Key low pass filter. The circuit is readily switched between single-ended and double-ended output, while maintaining comparable noise and gain characteristics. The gain of the circuit is readily adjustable over a range of gains. The circuit also provides a relatively low output impedance (approximately 100 ohms balanced, 50 ohms unbalanced) so as to enable driving typical devices over a substantial length of cable without undue high frequency signal loss.
The input signal to the circuit is in differential form, and is applied across a first pair of single-ended operational amplifiers each having a fixed resistor, and one or more switched resistors, in the feedback loop between the output terminal of the amplifier and the inverting input thereof. The input signal is coupled to the amplifier through a DC-blocking capacitor and a series-connected gain-setting resistor. The feedback resistors, in combination with the gain-setting resistors, establish the gain of the amplifier. A compensating capacitor in the feedback loop in parallel with the gain-setting resistors prevents oscillation and reduces high frequency noise, as well as reducing slew-rate-limit-induced distortion.
The input stage maintains a relatively constant gain-bandwidth product as the gain of the circuit is switched. For example, in one embodiment in accordance with the invention, I obtained a cutoff frequency (xe2x88x923 dB point) for the first stage output of 196 kHz with a gain of 2.87, resulting in a gain-bandwidth product of 562 kHz, and a cutoff frequency of 805 kHz with a gain of 0.701, resulting in a gain-bandwidth product of 564 kHz, essentially the same.
The output of each of the first-stage amplifiers is applied in a cross-coupled manner to complementary inputs of a pair of differential amplifiers (xe2x80x9cdoubly differentialxe2x80x9d) of the second-stage dual Sallen-Key filter in such a manner that the outputs of the first stage are summed in-phase in one section of the filter and out-of-phase in the other. This ensures that a high signal-to-noise ratio is provided by the filter, regardless of whether the filter is used in a single-ended or a double-ended output mode. Selection of the mode is accomplished by means of a pair of relays at the output of the second-stage amplifiers. The relays are configured such that each has a series arm and a shunt arm, the relays each being energizable to connect its series arm to a corresponding output terminal for signal output as desired, and being deenergizable to connect the corresponding output terminal to ground via the shunt arm, to thereby mute the output terminal. The negative output can be grounded in single-ended mode one only a single relay and with a gain difference of 6 db.